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Mutant monkeys get hooked on a drug called mum

A gene mutation determines whether or not macaque infants make a fuss when their mothers are missing, say researchers. A similar mutation has been linked to alcoholism and drug abuse in humans.

The work could explain why some children are cry babies and some are more independent, says Christina Barr, a neuroscientist at the National Institute of Alcohol Abuse and Alcoholism in Bethesda, Maryland, US, who led the study.

“There are some kids that go with the flow and there are some that are very reluctant [to leave their mothers],” she says.

When infants and young animals spend time with their mothers, their bodies release natural drugs called opioids. Molecular receptors on brain cells sop up these chemicals and provide temporary feelings of pleasure.

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Crying gene

Opiates like morphine act via the same receptors, and minute doses of the drug â” not enough to sedate â” quieten young macaques after their mothers leave.

Previous research has suggested that 25 to 30% of macaques have a genetic mutation in the mu-opioid receptor that makes them more sensitive to the brain chemicals.

To determine whether the mutation affects maternal attachment, Barr and her colleague Steve Suomi observed 97 six-month-old macaques after they were separated from their mothers as part of a natural weaning process for several four-day stretches. The monkeys were born in captivity.

All the infants howled the first time they faced life without their mother, but most of the animals grew accustomed to the separation and made less noise the next time they were separated.

Happier together

Not so for the animals with one or two copies of the opioid-enhancing mutation. Those monkeys continued to cry out for their absent mothers.

These monkeys also seemed to cherish reunions. After each separation they tended to spend more time with their mothers, whereas macaques without the mutation didn’t make up for lost time. “These animals were much more likely to want to stay in social contact with their mothers,” says Barr.

She thinks the mutation might keep animals in the wild safe from predators and other dangers when their mothers are away. “If you’re going to be the animal that gives up really quickly, maybe she’ll never find you, but if you keep crying she’ll hear you and come get you,” she says.

Drunken monkeys

The same mutation also adds to the high of alcohol and drugs, Barr says. She has already found that given the chance, male macaques with the opioid receptor mutation drink more alcohol than normal monkeys. A mutation common in humans provides the same opioid kick and is also linked to alcoholism and drug abuse.

“What is needed now is work at the human level to see if attachment style also varies,” says Jaak Panksepp, a neuroscientist at Washington State University in Pullman, US. Barr agrees&colon; “Early in evolutionary history there could have been some additional value to having a kid that was more attached,” she says. “Now that we have drugs and alcohol, it can make us more vulnerable to addiction.”